Default morphological operations dilate and erode are indeed working in binary mode (Dilation (morphology) - Wikipedia), this is the most usual way dilation/erosion with a structuring element are defined.
But there exists also a real mode which makes them work with float-valued kernels (Dilation (morphology) - Wikipedia). This mode can be activated by setting the argument is_real to 1 when calling those operators.
E.g.:
gmic sp tiger 32,32 gaussian. 20% n. 100,255 dilate.. .,1,1
Same for erode:
